Is this omega pocket watch original?

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Totally legit Omega in a very nice silver Art Nouveau case.
Dial with Empire numerals and hands match the period.
Serial number dates the watch to about 1920 or so.
Possibly a caliber 17''' L ?

@OMTOM @François Pépin may shed more light?
 
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Niello decoration on the case back. The case is .900 (coin) silver. A quality watch. Surprisingly only 15-jewel movement. Enamel dial looks in nice shape.
 
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Legit, yes, and not so common! Nice find!

The movement seems to have a nice silver finish, maybe to match the case, which is unusual for those 19' series like calibres - they usually have a gold finish unlike the later calibers such as the 35,5.

What is the size of the movement?